It is a well known fact that the aforesaid reaction yields, additionally to caprolactame, small amounts of non-cyclic amides, such as acetamide, popionamide, butyramide, n-valeramide, benzamide, tetrahydrobenzamide, hexahydrobenzamide, and moreover, still in small amounts, aliphatic and aromatic cyclocarboxylic acids, epsilon-aminocapronic acid, and unsaturated oxidable substances.
Several methods of purifying caprolactame are known in the art. Thus, for example, when effectuated by reacting nitrosyl compounds with carbocyclic derivatives, the purification of caprolactame (following its extraction from the sulphuric reaction mass and known steps for separating raw caprolactame) is substantially carried out, according to current practice, by means of two separate and discrete chemical treatments, a first one directed to destroying the amides, and a second one directed to reducing or eliminating the easily oxidable substances. More specifically, these treatments consist of subjecting firstly the raw caprolactame to the action of sodium hypochlorite (conversion of the amides into more volatile amines), and subsequently to an oxidizing action, e.g. with potassium permanganate or ozone (decomposition of the oxidable substances); the product, after undergoing that treatment, is subjected to distillations, possibly under vacuum conditions, without rectification, thereby a "polymerization grade" caprolactame is obtained. Said chemical treatments, however, have the following drawbacks:
(1) they are expensive;
(2) they yield impurities of a different nature (such as chloro-lactames, for instance) which, although only some ppm, result in contamination of the caprolactame;
(3) the treatments with hypochlorite-permanganate, respectively hypochlorite-ozone, are not absolutely selective, even when the very high caprolactame-byproducts ratio is taken into account, thereby part of the caprolactame is destroyed by these chemicals, with an attendant appreciable decrease of the yield.